An analysis of the changing social bases of America's political parties: 1952–2008

•The social bases of both the Republican and Democratic Party have changed considerably.•The Democratic Party is benefitting from the support of several groups that are increasing in size.•The Republican Party has remained competitive due to increasing group loyalty and turnout.•Demographic changes will force the Republicans to incorporate new groups to remain competitive.

In this article I address two interrelated questions: have the group bases of the American political parties changed over time and what factors have lead to the observed changes? I determine social group memberships significantly influence individual partisanship with a multivariate analysis using 56 years of ANES data. I then measure how many votes each politically relevant social group contributed to the party coalitions in each presidential election from 1952 to 2008. I discuss how group contributions have changed over time and establish the demographic and behavioral causes of group contribution change. I find that the party coalitions have been restructured as a result of groups' changing voting behavior and the changing ratio of groups in the electorate.